The present invention relates in general to a circuit arrangement for overvoltage protection of a power transistor for controlling an inductive load, such as, for example, the drive coil of a magnetic drive.
A circuit arrangement for the overvoltage protection of a power transistor for pulse-width control of a motor winding as inductive load is known from European Patent Application EP-A2-0 331 251. This protection circuit contains a first varistor in parallel with the input of a rectifier as well as a second varistor in parallel with the switching path of a power transistor which, along with the motor winding and a limiting resistor, is connected to the output of the rectifier. The first varistor, within the limits of its thermal loading capacity, provides the coarse protection of the entire circuit whereas the fine protection for the power transistor is taken care of by the second varistor, a parallel arrangement composed of a diode and a resistor and preferably connected in series to second the varistor absorbing the energy caused by the limiting second varistor. Under its limiting level, a noticeable transverse current flows through the second varistor, which, on one hand, results in a not inconsiderable power loss and, on the other hand, permits no or at least only a considerably limited pulse-width control of the load current in certain applications and conditions, in particular, in connection with drive coils of magnetic drives in the holding mode.
To avoid a continuous current through a varistor, Japanese Patent Application JP-A-05 207647 proposes a circuit arrangement for the overvoltage protection of a load which is connected to a DC voltage supply, in which both a first series connection of a varistor and the switching path of a bipolar switching transistor and a second series connection of a Z-diode and a capacitor are arranged in parallel with the load, the inner connection point of the second series connection being connected to the gate electrode of the switching transistor via a limiting resistor. Here, the threshold value of the overvoltage on the supply side for activating the voltage limitation disadvantageously depends on the power supply factor of the switching transistor and, in addition, is noticeably delayed by the capacitor. Analog disadvantages exist in connection with the undershooting of the threshold value.
Therefore, the object of the present invention is to provide low power loss overvoltage protection for a power transistor which permits in an unlimited manner.
The present invention provides an apparatus for overvoltage protection of a power transistor, the power transistor for controlling an inductive load connected in series with a switching path of the power transistor. The apparatus includes a first varistor bridging supply connections and a second varistor connected in series with a switching path of a switching transistor. The second varistor and the switching transistor are together connected in parallel with the switching path of the power transistor. A Zener diode is connected in series with an ohmic voltage divider, a divided voltage of the ohmic voltage divider controlling the switching transistor. The Zener diode and the ohmic voltage divider are together connected in parallel with the inductive load and the switching path of the power transistor together.
When a high overvoltage occurs at the supply connections, for example, due to the action of a lightning, the overvoltage is limited by the first varistor to the level of its terminal voltage in a usual manner. A further voltage limitation for the power transistor is accomplished in such a manner that, as a result of the Z-diode (Zener diode) becoming conductive, the switching transistor is enabled and, consequently, the blocking voltage for the switching path of the power transistor is limited to the terminal voltage level of the second varistor. The terminal voltage of the second varistor must be selected to be both considerably lower than the terminal voltage of the first varistor and, with sufficient reliability, to be lower than the maximum permissible blocking voltage of the power transistor. Due to the resistive component of the inductive load, the current is sufficiently limited by the switching transistor. Below the breakdown voltage of the Z-diode, as a result of the blocked switching transistor, no parasitic current occurs parallel to the controlling power transistor.